1. Field of the Invention
The present invention relates to a control apparatus for an AC motor, and more particularly, it relates to a technology for estimating a constant of the AC motor.
2. Description of the Related Art
Heretofore, as a control apparatus for an AC motor, to which a control method is applied, there has been available a control apparatus having such a constitution as shown in FIG. 3. In FIG. 3, a reference numeral 1 denotes an AC power source; 2 a converter; 3 a smoothing capacitor; 4 an inverter; 5 a current detector; 6 an induction motor; 11 a speed control unit; 12a a voltage operation unit; 13 a voltage conversion unit; 14 a q-axis current control unit; 15 a d-axis current control unit; 16 a correction voltage operation unit; 17 a d-axis secondary magnetic flux command unit; 18a a frequency operation unit; 19 an integrator; 20 a current conversion unit; and 21a a speed estimation unit.
An AC voltage outputted from the AC power source 1 is rectified by the converter 2, and smoothed by the smoothing capacitor 3 to be converted into a DC voltage. The DC voltage thus obtained by the conversion is converted into an AC voltage by the inverter 4 in accordance with a U-phase voltage command Vu*, a V-phase voltage command Vv* and a W-phase voltage command Vw*, which are all outputs from the voltage conversion unit 13. Then, the induction motor 6 is driven. A U-phase current Iu and a W-phase current Iw flowing through the induction motor 6 are detected by the current detector 5. The detected currents Iu and Iw are subjected to rotational coordinate transformation at the current conversion unit 20 based on a phase xcex8, which is an output from the integrator 19, and then converted into a d-shaft current Id and a q-axis current Iq as the components of a dq rotational coordinate system.
On the other hand, the speed control unit 11 controls a q-axis current command Iq* in such a way as to cause an estimated speed xcfx89r{circumflex over ( )} outputted from the speed estimation unit 21a to coincide with a speed command xcfx89r* provided from an external unit. At the q-axis current control unit 14, a q-axis current control quantity xcex94Vqo is outputted based on an expression (1) using a q-axis current command Iq* and a q-axis current Iq outputted from the speed control unit 11. In the expression (1) below, Kpcq and Kicq represent control gains, and s represents a differential operator.                               Δ          ⁢                      xe2x80x83                    ⁢          Vq0                =                              (                          Kpcq              +                              Kicq                s                                      )                    ·                      (                                          Iq                *                            -                              0                ⁢                q                                      )                                              (        1        )            
At the d-axis current control unit 15, a d-axis current control quantity xcex94Vd0 is outputted based on an expression (2) using a d-axis current command Id* and a d-axis current Id provided from the external unit. In the expression (2) below, Kpcd and Kicd represent control gains.                               Δ          ⁢                      xe2x80x83                    ⁢          Vd0                =                              (                          Kpcd              +                              Kicd                s                                      )                    ·                      (                                          Id                *                            -              Id                        )                                              (        2        )            
At the correction voltage operation unit 16, a d-axis voltage correction quantity xcex94Vd and a q-axis voltage correction quantity xcex94Vq are computed based on an expression (3) using the d-axis current control quantity xcex94Vd0 and the q-axis current control quantity xcex94Vq0. In the expression (3) below, Kd and Kq represent control gains.                               (                                                                      Δ                  ⁢                                      xe2x80x83                                    ⁢                  Vd                                                                                                      Δ                  ⁢                                      xe2x80x83                                    ⁢                  Vq                                                              )                =                              (                          xe2x80x83                        ⁢                                                            Kd                                                  0                                                                              Kq                                                  1                                                      ⁢                          xe2x80x83                        )                    ·                      (                                                                                Δ                    ⁢                                          xe2x80x83                                        ⁢                    Vd0                                                                                                                    Δ                    ⁢                                          xe2x80x83                                        ⁢                    Vq0                                                                        )                                              (        3        )            
At the voltage operation unit 12a, d-axis and q-axis voltage commands Vd* and Vq* as voltage commands in the dq rotational coordinate system are computed based on an expression (4) using the d-axis current command Id*, the q-axis current command Iq*, a d-axis secondary magnetic flux command xcfx86d* outputted from the d-axis secondary magnetic flux command unit 17, a frequency command value xcfx891* outputted from the frequency operation unit 18, the d-axis voltage correction quantity xcex94Vd and the q-axis voltage correction quantity xcex94Vq. In the expression (4), R1* represents a primary resistance set value; L"sgr"* a leakage inductance set value converted to the primary side; and Km an inductance ratio calculated from mutual inductance M* and secondary inductance L2* based on an expression (5).                                                                         (                                                                                                    Vd                        *                                                                                                                                                Vq                        *                                                                                            )                            =                              xe2x80x83                            ⁢                                                                    (                                          xe2x80x83                                        ⁢                                                                                                                        R1                            *                                                                                                                                                                                                                                ω                                  ⁢                                  1                                                                *                                                            ·                              L                                                        ⁢                                                          xe2x80x83                                                        ⁢                                                          σ                              *                                                                                                                                                                                                                                                                                                        ω                                  ⁢                                  1                                                                *                                                            ·                              L                                                        ⁢                                                          xe2x80x83                                                        ⁢                                                          σ                              *                                                                                                                                                            R1                            *                                                                                                                ⁢                                          xe2x80x83                                        )                                    ·                                      (                                                                                                                        Id                            *                                                                                                                                                                            Iq                            *                                                                                                                )                                                  +                                                                                                        xe2x80x83                            ⁢                                                (                                                                                    0                                                                                                                                                                                                                                ω                                ⁢                                1                                                            *                                                        ·                                                          Km                              *                                                        ·                                                          xe2x80x83                                                        ⁢                            φ                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      d                            *                                                                                                                                )                                +                                  (                                                                                                              Δ                          ⁢                                                      xe2x80x83                                                    ⁢                          Vd                                                                                                                                                              Δ                          ⁢                                                      xe2x80x83                                                    ⁢                          Vq                                                                                                      )                                                                                        (        4        )                                          Km          *                =                              M            *                                L2            *                                              (        5        )            
The d-axis and q-axis voltage commands Vd* and Vq* computed at the voltage operation unit 12a are subjected to transformation from a q-axis rotational coordinate axis to a fixed coordinate axis by the voltage conversion unit 13 based on a phase xcex8 outputted from the integrator 19, and converted into a U-phase voltage command Vu*, a V-phase voltage command Vv*, and a W-phase voltage command Vw*. Then, voltage control is carried out.
A d-axis secondary magnetic flux command xcfx86d* is computed from the d-axis current command Id* at the d-axis secondary magnetic flux command unit 17 based on an expression (6).
xcfx86d*=M*xc2x7Id*xe2x80x83xe2x80x83(6) 
At the frequency control unit 18, a frequency command value xcfx891* is computed based on an expression (7) using the q-axis current command Id*, the d-axis secondary magnetic flux command xcfx86d*, and a speed estimated value xcfx89r{circumflex over ( )} outputted from the speed estimation unit 21a. In the expression (7) below, R2* represents a secondary resistance set value.                                           ω            ⁢            1                    *                =                              ω            ⁢                          xe2x80x83                        ⁢                          r              ^                                +                                                    R2                *                            ·                              Km                *                            ·                              Iq                *                                                    φ              ⁢                              xe2x80x83                            ⁢                              d                *                                                                        (        7        )            
At the integrator 19, a phase xcex8 is calculated by integrating the frequency command value xcfx891*.
Furthermore, at the speed estimation unit 21a, a speed estimated value xcfx89r{circumflex over ( )} is computed based on an expression (8) using the q-axis current Iq and the d-axis secondary magnetic flux command xcfx86d*. In the expression (8) below, T1 represents a control constant used to decide estimation response.                                                                         ω                ⁢                                  xe2x80x83                                ⁢                                  r                  ^                                            =                              xe2x80x83                            ⁢                                                1                                      1                    +                                          T1                      ·                      s                                                                      xc3x97                                  1                                                                                    Km                        *                                            ·                      φ                                        ⁢                                          xe2x80x83                                        ⁢                                          d                      *                                                                      xc3x97                                  {                                                                                                              Km                          *                                                ·                        φ                                            ⁢                                              xe2x80x83                                            ⁢                                                                        d                          *                                                ·                        ω                                            ⁢                                              xe2x80x83                                            ⁢                                              1                        *                                                              +                                                                                                                                          xe2x80x83                            ⁢                                                                    R1                    *                                    ·                                      Iq                    *                                                  -                                                      (                                                                  R1                        *                                            +                                              Km                                                                              *                            2                                                    ⁢                                                      ·                                                          R2                              *                                                                                                                          +                                              L                        ⁢                                                  xe2x80x83                                                ⁢                                                                              σ                            *                                                    ·                          s                                                                                      )                                    ·                                                                                                                        xe2x80x83                            ⁢                                                Iq                  +                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                    Vq0                                                  }                                                                        (        8        )            
According to the control system conFIG.ured in the foregoing manner, as long as there is coincidence between a motor constant used for the primary resistance of the induction motor 6 and a motor constant used for the voltage operation unit 12a, the frequency operation unit 18a and the speed estimation unit 21a, then the secondary magnetic flux of the induction motor 6 is controlled to be constant, and the q-axis current Iq is set proportional to torque. Accordingly, a good control characteristic can be obtained.
As such a control method, there have been available control methods respectively described in Japanese Patent Application Laid-open Nos. 105580/1994, 284771/1994 and 317698/1996.
As a method for estimating a constant of the induction motor, there has been available a method described in Japanese Patent Application Laid-Open No. 80100/1996. According to the constant estimation method disclosed therein, when an actual value of a primary magnetic flux coincides with a set value obtained by the product of primary self-inductance and an exciting current, a constant of the induction motor can be estimated based on an error current of zero. Another method available for estimating a constant of the induction motor is one described in Japanese Patent Application Laid-Open No. 191699/1997. According to the constant estimation method disclosed therein, primary resistance and leakage inductance are estimated according to changes in a d-axis current command value and a d-axis current. The primary resistance is identified when at least one of a frequency command value and a q-axis current is equal to a predetermined value or lower. The leakage inductance is identified when a frequency command value and a q-axis current are both equal to predetermined values or higher.
In the control method of the induction motor available in the conventional art, the induction motor is controlled based on the constant of the induction motor set in the control apparatus. However, the constant of the induction motor may vary due to the temperature of the induction motor. When there is a change in a temperature during running, the constant is also varied. When a d-axis current command is reduced in order to perform control in a wide speed range, the constant may vary because of the effect of magnetic flux saturation inside the induction motor. If there is a difference between the constant of the induction motor set in the control apparatus and an actual constant because of such constant fluctuation, then control performance may be deteriorated or an unstable phenomenon may occur.
The conventional method described in Japanese Patent Application Laid-Open No. 80100/1996 cannot be applied to the control system for setting a secondary magnetic flux constant. It is because the method disclosed therein is designed to make a primary magnetic flux coincident with a reference value. Moreover, the method described in Japanese Patent Application Laid-Open No. 191699/1997 is designed to switch constants to be estimated between the frequency command value and the q-axis current. Consequently, shocks may occur in the vicinity of a switched frequency during acceleration/deceleration. A term regarding a speed electromotive force by a magnetic flux cannot be estimated.
It is an object of the present invention to provide a control apparatus of an induction motor, capable of suppressing the deterioration of control performance caused by the setting errors or the fluctuation of constants of the induction motor during running, the constants being used for control in a control method for setting a secondary magnetic flux constant, by estimating primary resistance, an inductance ratio or primary self-inductance, and leakage inductance among such constants.
In order to achieve the foregoing object, according to the present invention, there is provided a control apparatus of an induction motor, comprising: a detector for detecting a primary current of the induction motor; primary current component detecting means for detecting an exciting current and a torque current from the current detected by the detector; correcting means for outputting a correction voltage to set zero a difference between an exciting current command value and the exciting current outputted from the primary current detecting means; frequency voltage control means for outputting a frequency command and a voltage command based on an output of the correcting means, primary resistance, an inductance ratio between mutual inductance and secondary self-inductance, and leakage inductance of the induction motor; and at least one selected from fist to third constant correcting means, the first constant correcting means being for correcting the primary resistance based on a quantity obtained by multiplying a quantity of a voltage command outputted from the frequency voltage control means by the output of the correcting means, the quantity of the voltage command being a result of partial differentiation with the primary resistance, the second constant correcting means for correcting the leakage inductance based on a quantity obtained by multiplying a quantity of the voltage command outputted from the frequency voltage control means by the output of the correcting means, the quantity of the voltage command being a result of partial differentiation with the leakage inductance, and the third constant correcting means for correcting the inductance ratio based on a quantity obtained by multiplying a quantity of the voltage command outputted from the frequency voltage control means by the output of the correcting means, the quantity of the voltage command being a result of partial differentiation with the inductance ratio. Thus, among constants of the induction motor used for control, primary resistance, an inductance ratio or primary self-inductance, and leakage inductance can be set optimal according to a running state, making it possible to suppress the deterioration of control performance caused by the setting errors or the fluctuation of these constants during running.